Since you're on 4.2.10 you won't be able to check Cydia for your saved SHSHs, but instead you can download the latest version of TinyUmbrella from
The Firmware Umbrella - TinyUmbrella and then connect your device to your computer and highlight it in the left pane. Go to the advanced tab and check the "Request SHSH from Cydia" box if it isn't already. Then click "Save SHSH" at the top.
All your available SHSHs will show up under the "general" tab. If you have 4.2.8 saved you can downgrade and do an untethered jailbreak with JBME